In U.S. Pat. Nos. 4,622,423 and 4,788,333, Burke discloses a two-step process for converting butadiene to adipic acid which gives high yields of this important diacid. The first step is the hydrocarboxylation of butadiene to form 3-pentenoic acid. The second step is the hydrocarboxylation of 3-pentenoic acid with carbon monoxide and water in the presence of a rhodium-containing catalyst, an iodide promoter and certain inert halocarbon solvents, e.g., methylene chloride.
It is disclosed in U.S. Pat. No. 4,788,333 that .gamma.-valerolactone is a significant by-product in the hydrocarboxylation of butadiene. It has also been found that .alpha.-methyl-.gamma.-butyrolactone is a minor (&lt;1%) by-product of this reaction. Both .gamma.-valerolactone and .alpha.-methyl-.gamma.-butyrolactone are inert towards further carbonylation (or hydrocarboxylation) under the reaction conditions which are effective for the hydrocarboxylation of 3-pentenoic acid. Since these by-products represent a yield-loss, it would be useful to find a process for converting these lactones to adipic acid.
Japanese Published Pat. application, Sado et al., 92,913/1979 discloses the preparation of dicarboxylic acids, by reacting lactones and carbon monoxide in the presence of platinum group catalysts, with iodine compounds as promoters. In Example 3, .gamma.-valerolactone is carbonylated in the presence of a rhodium catalyst to give the expected 2-methylglutaric acid in low yield (20.5 %) and a smaller amount of the unbranched adipic acid (5.8%). It has now been found that certain branched lactones can be carbonylated to form substantial amounts of the unexpected, unbranched dicarboxylic acids.